(1) Field of the Invention
The present invention relates to a sheet feeder and also to a document reader and an image forming apparatus each having the sheet feeder.
(2) Related Art
An electrophotographic image forming apparatus forms an image by transferring a toner image formed on a surface of a photoreceptor to a recording sheet transported by a sheet feeder.
The sheet feeder picks up a recording sheet from a paper feed cassette using a pick-up roller, and, subsequently, transports the recording sheet to, via a separation roller, a transfer position at a proper timing determined by a pair of registration rollers. At the transfer position, the toner image is transferred to the recording sheet.
With respect to this type of image forming apparatus, in order to achieve high productivity when consecutively forming images on a plurality of recording sheets, it is desirable that a sheet interval (i.e., the interval between the trailing edge of a recording sheet fed earlier (hereinafter, referred to as a “preceding sheet”) and the leading edge of the recording sheet fed subsequently to the preceding sheet is as short as possible.
Note that, when the pick-up roller picks up the uppermost recording sheet from a stack of recording sheets stored in the paper feed cassette, the second uppermost recording sheet occasionally picked up together with the uppermost one at a time. The second uppermost recording sheet (hereinafter, referred to as the “following sheet”) is forcibly separated from the preceding sheet (i.e., the uppermost recording sheet) by the separation roller. However, in some cases, the sheets separate spontaneously at a position between the pick-up roller and the separation roller. Accordingly, the position where the following sheet is separated from the preceding sheet can be different each time.
One approach to controlling the sheet interval to be constant is, for example, to determine the timing when the following sheet is transport based on the separation timing (i.e., the timing when the preceding sheet is forwarded and the trailing edge thereof is separated from the leading edge of the following sheet). This approach requires accurate detection of the timing when the preceding sheet is separated from the following sheet.
For example, Japanese Patent Application Publication No. 2012-236684 discloses the following: light emitted by a single light emitting element travels through a light diffusion plate, and irradiates a predetermined section along the direction of a transport path, the section starting from the vicinity of a pick-up position where the pick-up roller comes into contact with a recording sheet; the light with which the predetermined section is irradiated is converged, by a light converging plate, onto a light receiving surface of a single light receiving element located on one side of the transport path opposite to the other side on which the light source is located;
When a gap occurs as a result of the separation between the preceding sheet and the following sheet, the output from the light receiving element varies due to the light passing through the gap, which allows for the detection of the timing of separation between the preceding sheet and the following sheet within the predetermined section.
However, the gap detection method according to Japanese Patent Application Publication No. 2012-236684 requires a space large enough to arrange both the light diffusion plate and the light converging plate as optical elements. This makes it difficult to miniaturize the image forming apparatus. In addition, these optical elements are not inexpensive, and therefore increase the cost of the apparatus.
In view of the above, the inventors of the present invention devised a structure in which: a light emitting element emitting diffused light and a light receiving element are disposed on a first side of the sheet transport path; and a converging reflector is disposed on the other side of the sheet transport path for converging, onto the light receiving element, light that is emitted by the light emitting element and reaches the converging reflector by travelling across a predetermined section of the sheet transport path from the pick-up position of the pick-up roller to the separation roller.
When a gap occurs between the preceding sheet and the following sheet, the light passing through the gap is reflected by the converging reflector and is subsequently received by the light receiving element after passing through the gap again. The timing when the separation occurs between the preceding sheet and the following sheet is detected based on the variation in the output from the light receiving element.
Since the above-mentioned structure requires no optical element other than the converging reflector, this structure contributes to space saving, and achieves cost reduction.
However, the inventors found by repeating experiments with prototypes that this structure might cause detection error depending on the type of a sheet.